I recently received a kiln. I got approval to install it in my classroom. I scoured the catalogues for clay and glaze and got a grant for the supplies. The clay says "fires best at cone 04 and 05 not higher than cone 3." The glaze is also low fire and the directions say..... 2-3 coats on cone 04 bisque and then let dry and fire to shelf cone 06. I thought 04 was higher than 06? In fact just after the cone listing 06 states (999 degrees Celsius) and The cone 04 listed a higher temperature celsius. Shouldn't be the other way around? The higher temperature is when vitrification takes place???

At home I fire 04 bisque and then cone 6 glaze and it all works but that's all I have done. Am I missing something. Did I purchase the wrong stuff? Any insight would be appreciated. Perhaps they are labeled wrong?Thanks for any input.

The labels are correct, as are you in regards to Cone 04 being a higher temperature than 06.

Low fire clay does not vitrify like mid to high fire clays, which is why they require glaze/ underglaze to seal them. Otherwise, they would still be porous and liquids would slowly seep through.

So they second firing does not need to be as hot, as it just needs to melt the glaze.

Thank you. I am relieved wasn't sure how I was going to explain that I needed to return items because I got the wrong thing. I must have read the catalogue descriptions 100 times and talked to the woman on the phone and she assured me I was ordering the right stuff but when I saw the labels I got a bit worried.Renee

For earthenware, the higher 04 bisque allows more of the nasty ingredients to burn out before you glaze fire. Glazing is not a substitute for a vitrified clay body; a vitrified clay body will be water-tight without any glazes; the same is not true for an unvitrified clay body with glaze.

This is wonderful. Be sure to pay close attention to safety and of course you know that you must ventilate. Also there are some chemicals that are unsafe for children and child bearing age women to be exposed to such as cobalt, and copper and there are others please be sure to get all of the information you can so that you and the kids will be safe.

Amaco has lots of information for teachers working with clay and children. Please avail yourself of it. Be safe and do good work and have fun. I am so happy for you and especially for the kids. Don't forget at some point to have a Pottery Show and invite the whole school, near by schools, PTA, parents and local politicians. Make up a program and cards to identify the children whose work is on display and thank all the folks who gave the funding. God bless you all.

John 3:16
"For God so loved the world that He gave His only begotten Son, that whosoever believeth in Him should not perish but have everlasting life".

Yeah, it never hurts to check the labels, for safety information. I almost exclusively, use Amaco glazes. I've only had a couple, that had relatively toxic materials in them (It was a really bright orange). Since then, I haven't had one, with a similar issue. Some are not food safe, but that is because of the texture they produce, when fired.

Now, I do still have some glazes from Minnesota Clay. Nearly all of those, have some type of soluble copper, cobalt, etc. Once they run out, I shan't be replacing them. Speaking of, would such a glaze be food safe, once it is fired? The label doesn't indicate either way.

This is wonderful. Be sure to pay close attention to safety and of course you know that you must ventilate. Also there are some chemicals that are unsafe for children and child bearing age women to be exposed to such as cobalt, and copper and there are others please be sure to get all of the information you can so that you and the kids will be safe. Amaco has lots of information for teachers working with clay and children. Please avail yourself of it. Be safe and do good work and have fun. I am so happy for you and especially for the kids. Don't forget at some point to have a Pottery Show and invite the whole school, near by schools, PTA, parents and local politicians. Make up a program and cards to identify the children whose work is on display and thank all the folks who gave the funding. God bless you all.

Thank you for your advice and I fortunately have a closet that was originally out fitted for a kiln and since I'm on the good side of the maintenance crew they were able to dig out a vent that they had stored. The closet has a heavy door that does not unlock you can only open it with a key so no one can get in unless they have a key. In that way it will be kid proof.

I spent quite some time on the catalogue and on the phone making sure I had glazes that met the standards Massachusetts allows in the school. I don't want to risk anything at all. I figured the directions on the bottle were correct but I didn't understand that vitrification can be before glazing as I had not done that at home with my own kiln and glazes. Thanks again I am so looking forward to doing this with my students.

It sounds like you have earthenware clay, which WON'T vitrify. That's one of the things that differentiates earthenware and stoneware. You'll have to do some tests, but I think it'll work just fine for student work with the cone 04 bisque to burn out the gunk and cone 06 glaze fire just to melt the glaze. If it IS earthenware, you might work on steering your students to decorative pieces rather than functional pieces to avoid issues with the fact that the glaze is whats doing all the waterproofing.

Standard sells an earthenware that is 1% absorbent at cone 02 . . . pretty vitrified. With most earthenwares, if you fire to vitrification, you have to go high enough that you lose the terra cotta color.

C/06 Ox. C/02 Ox.

417 RED EARTHENWARE CLAY (Cone 06-02)

Low fire body that contains fire clay and a small amount of fine grog. Good for Majolica technique. Superior throwing body. Shrinkage: 6% at C/06, 11% at C/02Absorption: 5.3% at C/06, 1% at C/02

It sounds like you have earthenware clay, which WON'T vitrify. That's one of the things that differentiates earthenware and stoneware. You'll have to do some tests, but I think it'll work just fine for student work with the cone 04 bisque to burn out the gunk and cone 06 glaze fire just to melt the glaze. If it IS earthenware, you might work on steering your students to decorative pieces rather than functional pieces to avoid issues with the fact that the glaze is whats doing all the waterproofing.

But as long as the objects are glazed properly, why can't they be functional?

It sounds like you have earthenware clay, which WON'T vitrify. That's one of the things that differentiates earthenware and stoneware. You'll have to do some tests, but I think it'll work just fine for student work with the cone 04 bisque to burn out the gunk and cone 06 glaze fire just to melt the glaze. If it IS earthenware, you might work on steering your students to decorative pieces rather than functional pieces to avoid issues with the fact that the glaze is whats doing all the waterproofing.

But as long as the objects are glazed properly, why can't they be functional?

Because glaze usually doesn't stop the pot from leaking (microscopic crazing). It may slow it down. You should always test new clays for leaking. It's not that hard to throw a small cylinder, fire it unglazed, and leave it overnight full of water on a sheet of paper. The slightest dampness means that your new clay fired to that cone leaks and most likely no glaze you use will stop that leak. If you don't do that you deserve to be sued when somebody puts your vase on their grand piano.

It sounds like you have earthenware clay, which WON'T vitrify. That's one of the things that differentiates earthenware and stoneware. You'll have to do some tests, but I think it'll work just fine for student work with the cone 04 bisque to burn out the gunk and cone 06 glaze fire just to melt the glaze. If it IS earthenware, you might work on steering your students to decorative pieces rather than functional pieces to avoid issues with the fact that the glaze is whats doing all the waterproofing.

But as long as the objects are glazed properly, why can't they be functional?

Because glaze usually doesn't stop the pot from leaking (microscopic crazing). It may slow it down. You should always test new clays for leaking. It's not that hard to throw a small cylinder, fire it unglazed, and leave it overnight full of water on a sheet of paper. The slightest dampness means that your new clay fired to that cone leaks and most likely no glaze you use will stop that leak. If you don't do that you deserve to be sued when somebody puts your vase on their grand piano.

Jim

Honestly, I've never had an issue with leakage, but I'll test some of my wares, just to be sure.

No worries about one of my pieces ruining a grand piano. I really don't sell anything, and the stuff I make and give as gifts, isn't meant to hold liquid for long.

Eh, there's a lot of factors, really, that will ultimately affect how 'functional' the work is. I'll admit, I'm not familiar with Standard earthenware clays, the little bit of earthenware I've worked with was not very useful for kitchen ware. I think I could have said it better by saying TEST the clay/glaze combo first and see if it will work for something like mugs, and if it turns out it won't, THEN steer the students toward something like flower pots where it won't matter as much if the piece seeps a bit.

I was basing the 'WON'T vitrify' on the firing range the OP gave, at cone 04 its probably NOT vitrified, but maybe it is at the upper end of cone 3. And students frequently don't get a good coat of glaze on work until their third or fourth piece.

ANYWAY you do it meisie, a test run using your planned firing schedules is a good idea, even if you're just firing a few pieces in a mostly empty kiln.

Having read all of the above, it's making me worry about my clay / glaze now. I am using smooth white earthenware - and mostly make decorative pieces. However, if I want to make some mugs, what would be the best bisque firing temperature and same for glaze fire. I generally use a mid range transparent glaze, coloured with small qty of oxide. (Copper carbonate, cobalt carbonate, red/black iron oxide,)Are these 'safe'?
All advice greatly welcomed.

Having read all of the above, it's making me worry about my clay / glaze now. I am using smooth white earthenware - and mostly make decorative pieces. However, if I want to make some mugs, what would be the best bisque firing temperature and same for glaze fire. I generally use a mid range transparent glaze, coloured with small qty of oxide. (Copper carbonate, cobalt carbonate, red/black iron oxide,)Are these 'safe'?All advice greatly welcomed.

Celia

It is morally reprehensible for a potter to sell pots used to serve food without testing the glaze for leaching and the clay for leaking. Of course, most of us would rather have a mug that leaks than a mug that poisons us. If a glaze contains any metallic colorants other than iron then it should either not be used as a liner or it should be tested for leaching. You can do a simple lemon test to eliminate an obvious leacher but only a lab test will detect more subtle leaching.

Eh, there's a lot of factors, really, that will ultimately affect how 'functional' the work is. I'll admit, I'm not familiar with Standard earthenware clays, the little bit of earthenware I've worked with was not very useful for kitchen ware. I think I could have said it better by saying TEST the clay/glaze combo first and see if it will work for something like mugs, and if it turns out it won't, THEN steer the students toward something like flower pots where it won't matter as much if the piece seeps a bit.

I was basing the 'WON'T vitrify' on the firing range the OP gave, at cone 04 its probably NOT vitrified, but maybe it is at the upper end of cone 3. And students frequently don't get a good coat of glaze on work until their third or fourth piece.

ANYWAY you do it meisie, a test run using your planned firing schedules is a good idea, even if you're just firing a few pieces in a mostly empty kiln.

To be honest I'm not too worried about functionality. I think most of the projects will be sculptural in nature. We are starting this after years of air dry clay so even a little seepage will be better than what we used to do. thanks for all your input.

Having read all of the above, it's making me worry about my clay / glaze now. I am using smooth white earthenware - and mostly make decorative pieces. However, if I want to make some mugs, what would be the best bisque firing temperature and same for glaze fire. I generally use a mid range transparent glaze, coloured with small qty of oxide. (Copper carbonate, cobalt carbonate, red/black iron oxide,)Are these 'safe'?All advice greatly welcomed.

Celia

It is morally reprehensible for a potter to sell pots used to serve food without testing the glaze for leaching and the clay for leaking. Of course, most of us would rather have a mug that leaks than a mug that poisons us. If a glaze contains any metallic colorants other than iron then it should either not be used as a liner or it should be tested for leaching. You can do a simple lemon test to eliminate an obvious leacher but only a lab test will detect more subtle leaching.

Jim

The glazes I use, are marked as food safe, by the company. I'm assuming, that would indicate, that leaching isn't isn't an issue. Yes, I agree, that I prefer my mugs leaky, than poisony.

Some of my classroom glazes have soluble copper and cadmium. They are leftovers from a previous teacher, and I'm using them until they are gone, but I don't allow students to use them, for anything that might be used with food or drink. How does the lemon test work?Also, how is it, that glazes used lead for so many years?

Having read all of the above, it's making me worry about my clay / glaze now. I am using smooth white earthenware - and mostly make decorative pieces. However, if I want to make some mugs, what would be the best bisque firing temperature and same for glaze fire. I generally use a mid range transparent glaze, coloured with small qty of oxide. (Copper carbonate, cobalt carbonate, red/black iron oxide,)Are these 'safe'?All advice greatly welcomed.

Celia

It is morally reprehensible for a potter to sell pots used to serve food without testing the glaze for leaching and the clay for leaking. Of course, most of us would rather have a mug that leaks than a mug that poisons us. If a glaze contains any metallic colorants other than iron then it should either not be used as a liner or it should be tested for leaching. You can do a simple lemon test to eliminate an obvious leacher but only a lab test will detect more subtle leaching.

Jim

The glazes I use, are marked as food safe, by the company. I'm assuming, that would indicate, that leaching isn't isn't an issue. Yes, I agree, that I prefer my mugs leaky, than poisony.

Some of my classroom glazes have soluble copper and cadmium. They are leftovers from a previous teacher, and I'm using them until they are gone, but I don't allow students to use them, for anything that might be used with food or drink. How does the lemon test work?Also, how is it, that glazes used lead for so many years?

Lemon test, though not accurate will give an indication of the durability of the glaze. Leave slice of lemon on a plate, let it sit there for a few days, rinse and compare the surface where the lemon was to the rest. Another test is to put a test tile partially submerged in a container of vinegar and allow to sit for a few days, then compare the vinegar exposed to unexposed. Try this also with a solution of dishwashing detergent(alkali). These two tests will give you some indication of how your glazes holdup to acid and alkali.

Benzine;
"How is it the glazes used lead?"
Well, sit down, young man, and I will tell you the story of lead in glaze.
The source of lead is the mineral "galena", found all over the world as a surface rock. The galena can be used straight as a glaze, and was dusted on the pots through a bag or an old sock. This was a job for the potter's wife, and many died from lead poisoning. People did not know that lead was poisonous, or if they did, they did tell their wives.[Sick, I know. I apologize]
The great thing with lead was that it made a beautiful glaze and a very reliable flux in low fire glazes. Lead glazes were common in schools right up into the sixties. Art teachers were also doing copper enameling in their art rooms without proper ventilating.
Lead is still used in glazes in third world countries, as it is easily available in car batteries, which are taken apart and pounded into a glaze.Be careful when buying souvenir pottery from Mexico and other countries, and don't use them with food.
The opinions expressed here are my own.
TJR.

Having read all of the above, it's making me worry about my clay / glaze now. I am using smooth white earthenware - and mostly make decorative pieces. However, if I want to make some mugs, what would be the best bisque firing temperature and same for glaze fire. I generally use a mid range transparent glaze, coloured with small qty of oxide. (Copper carbonate, cobalt carbonate, red/black iron oxide,)Are these 'safe'?All advice greatly welcomed.

Celia

It is morally reprehensible for a potter to sell pots used to serve food without testing the glaze for leaching and the clay for leaking. Of course, most of us would rather have a mug that leaks than a mug that poisons us. If a glaze contains any metallic colorants other than iron then it should either not be used as a liner or it should be tested for leaching. You can do a simple lemon test to eliminate an obvious leacher but only a lab test will detect more subtle leaching.

Jim

The glazes I use, are marked as food safe, by the company. I'm assuming, that would indicate, that leaching isn't isn't an issue. Yes, I agree, that I prefer my mugs leaky, than poisony.

Some of my classroom glazes have soluble copper and cadmium. They are leftovers from a previous teacher, and I'm using them until they are gone, but I don't allow students to use them, for anything that might be used with food or drink. How does the lemon test work?Also, how is it, that glazes used lead for so many years?

Lead is not all that deadly and it is even possible to use very small amounts safely in a glaze that is carefully fired, so it has been used on pots for many years. But, it's for the best that we now consider it better to not use it at all for dishes. It is still available, usually in frits, for glazes. Pres answered your question about the lemon test. Since most potters aren't going to go to the trouble and expense of sending pots to a lab to be tested, the lemon/vinegar tests are better than no testing at all, but even better is to not use any glaze for a liner that has any metalic colorant in it other than iron unless it is tested in a lab.